1. Field of the Invention
This invention relates to microprocessor devices, and in particular to a microcomputer system having multiple but mutually exclusive operative memory units.
2. Prior Art
In the past, digital systems have been fabricated from a multiplicity of logic circuits such as AND or OR gates, flip-flops and the like. One or more of such logic circuits are integrated onto a single semiconductor chip. These logic circuits are commonly referred to in the art as small scale integration. Numerous small scale integrated logic circuits are assembled onto a board containing printed circuit interconnections between each of the individual logic circuits, and numerous such assembled boards are electrically interconnected for constructing programmable digital systems. These digital systems have disadvantages of being large in size, requiring large amounts of power for operation, and are relatively slow in operation due to the physical distance between components.
More recently, typical circuits frequently employed in constructing digital systems have been integrated onto a single chip thereby mitigating somewhat the above-described disadvantages.
With recent improvements in integrated circuit technology, a still larger number of circuit components can be integrated onto a single semiconductor chip. Thus, larger portions of the digital systems can be combined onto a single semiconductor chip. However, this improvement in the integrated circuit art has brought about different types of problems. For example, integrated circuits for digital systems were custom-made for different applications, and the cost for such custom-made circuits is prohibitive. Also, with a larger number of circuits on a single semiconductor chip, a concomitant increase in the number of pin connections to the integrated circuit chip was required.
The more recent developments of microprocessor devices has made possible the replacement of numerous custom digital circuits with one or more such programmed microprocessor devices. A microprocessor system comprises a stored-program digital system much like the well-known large computer systems. However, programming of a microprocessor is different from programming a large computer system in that the microprocessor forms a part of a larger system in many instances, and it would be helpful to the designer to emulate the operation of the microprocessor system during design of the larger system in which it is employed.